This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. We have discovered that SgrAI, a sequence specific double stranded endonuclease, forms oligomers of the DNA bound form, which activate the enzyme's DNA cleavage activity, and also alter its sequence specificity. The oligomerization may serve to sequester activated enzymes away from the host genomic DNA, and may also serve to rapidly activate the enzyme to protect the host from invading phage. High resolution x-ray crystal structures show the structure of the unoligomerized form (a DNA bound enzyme dimer), and of a potential building block of the oligomer (two DNA bound enzyme dimers associated "head-to-head"). The "head-to-head" associated DNA bound enzyme dimers utilized domain swapping to stabilize, in part, the association, which is quite unusual and unexpected. We hope to characterize, using SAXS, the larger oligomer that is stabilized by binding to DNA that is longer than that used in the crystal structures.